To understand the mechanism of contraction in a muscle it is necessary to know how the contractile elements are organized and how the interaction of contractile proteins is triggered and controlled. In striated muscle differential extraction staining with fluorescent antibodies or with ferritin-labelled antibodies. The role of intracellular calcium ion concentration ((Ca 2 ion)i) in controlling contraction has also been elucidated mainly by observation of single fibers. Such experiments are now possible with smooth muscle due to the development of techniques for separating smooth muscles into isolated cells and feasible since a definite organization of contractile elements has been demonstrated. The purpose of the research is to apply the techniques used so successfully with striated muscle to single smooth muscle cells to determine the organization of contractile elements and the role of (Ca 2 ion)i. It is hoped that the information obtained will allow a comprehensive model of contraction for smooth muscle to be developed. Such information should provide a better basis for devising treatment for diseases, such as asthma and some types of hypertension, where controlling smooth muscle activity is a primary concern.